Electrical devices such as power capacitors, transformers, condensers, cables, circuit breakers and the like often utilize a dielectric fluid as an insulating and cooling medium. For their insulating function, dielectric fluids must have high electrical resistance, high dielectric strength, and low conductivity. In the cooling function, the fluids should have characteristics such as good heat transfer and dissipation, low freezing point and high boiling point. The fluid must have excellent resistance to decomposition over long periods of time and under severe operational conditions. The dielectric fluid must not decompose to form electrically conductive or corrosive materials. Most importantly, satisfactory dielectric fluid will also be nonflammable.
Many materials have previously been employed as dielectric fluid, including mineral oils, esters of organic acids, castor oil, aromatic hydrocarbons and alkylates thereof, and the like. Few of these materials display all of the requisite characteristics for a satisfactory dielectric. The halogenated hydrocarbons such as trichloroethylene and perchloroethylene have also been suggested as dielectric fluids, particularly in combination with other chlorinated ethylenes and chlorinated aromatic hydrocarbons. Such combinations are disclosed in U.S. Pat. Nos. 1,966,901 and 2,019,338.
More recently, the highly chlorinated hydrocarbons such as polychlorinated biphenyls have been widely used. While these materials are functionally advantageous, they are objectionable because of their toxicity and persistence in the environment. Therefore, dielectric fluids which are nontoxic, nonflammable, environmentally acceptable, economical and resistant to degradation have been actively sought.